Field of the Invention
This invention relates to a novel gene PpeGID1c and its role in controlling growth and elongation of peach vegetative tissues and to new methods of manipulating tree size by silencing PpeGID1c.
Description of the Relevant Art
Trees in agricultural production systems require unique management and horticultural practices. These include grafting, pruning, training, harvesting, and spraying strategies specifically designed to manage and/or accommodate tree size and structure. Such practices are both costly and labor intensive. For example, many fruit and nut trees utilize dwarfing rootstocks to control tree size. This requires additional material and labor costs. Genetic manipulation of tree size either through breeding or biotechnology offers a promising approach to minimize the efforts needed to optimize tree growth (Webster, T. 2002. Dwarfing Rootstocks: Past, Present and Future 35:67-72). To date, only a few studies have investigated ways to genetically manipulate tree growth rates and overall height. These studies have involved the manipulation of genes involved with Gibberellic Acid (GA) hormone levels and/or response signaling pathways. GA is crucial for many aspects of plant development and growth. In plum and poplar, high levels or over-expression of GA2Oxidase (GA2Ox) genes, which code for enzymes that convert active forms of GA to an inactive form, resulted in reduced vigor, stature, and, in some cases, extreme dwarfism (Busov et al. 2003. Plant Physiol. 132:1283-1291; El-Sharkawy et al. 2012. J. Exp. Bot. 63:1225-1239; Zawaski et al. 2011. Planta 234:1285-1298). The same phenomenon has been observed in many herbaceous species when GA2Ox genes are up-regulated (Appleford et al. 2007. J. Exp. Bot. 58:3213-3226; Dijkstra et al. 2008. Plant Cell Rep. 27:463-470; Sakamoto et al. 2001. Plant Physiol. 125:1508-1516; Sakamoto et al. 2004. Plant Physiol. 134:1642-1653; Wuddineh et al. 2015. Plant Biotechnol. J. 13:636-647; Otani et al. 2013. J. Plant Physiol. 170:1416-1423; Lee and Zeevaart. 2005. Plant Physiol. 138:243-254). Complementing these studies, high levels of GA2Oox, an enzyme for the biosynthesis of active GAs, are associated with increased height and branch diameter in pine trees (Park et al. 2014. Tree Physiol. 35:86-94). Additionally, overexpression of dominant negative mutant versions of a GA response repressor called DELLA such as repressor of gal-like (rgl) and ga insensitve (gai), led to dwarfism in both trees and herbaceous species (Zawaski et al., supra). A significant limitation of these methods for use in dwarfing fruit trees is that the GA hormone is required for proper flower and fruit development (Bulley et al. 2005. Plant Biotech. J. 3:215-223).
Breeding efforts using naturally occurring germplasm to reduce tree size and optimize shape have been carried out in a number of tree crops, particularly those for fruits and nuts. In peach, a handful of these traits have been genetically characterized including several dwarf traits, called dw, which have been used to breed new varieties with the potential to increase productivity and reduce labor costs. At least three dw loci, called dw, dw2, and dw3, have been reported as single recessive genes (Scorza et al. 2002. J. Am. Soc. Hortic. Sci. 127:254-261; Chaparro et al. 1994. Theor. Appl. Genet. 87:805-815; Shimada et al. 2000. J. Japanese Soc. Hortic. Sci. 69:536-542; Monet et al. 1988. Agronomie 5:727-731; Hansche, P. E. 1988. Hortscience 23:604-606). Peach trees homozygous for dw or dw2 display brachytic dwarfism (FIG. 1). They have extremely short internodes, thickened stems, reduced higher order branching, and elongated leaves. dw3 peach trees display a distinct dwarf phenotype marked by narrow branches and willowy leaves (Chaparro et al., supra). Several different sources of dw have been described (including ornamental types from Japan and China); however, no tests for allelism have been reported. One of the dw loci was genetically mapped by Shimada et al. (2000, supra) in a cross between the Japanese cultivars ‘Akame’ and the ornamental dwarf type ‘Jusietou’ and later positioned to the proximal end of linkage group 6 (Dirlewanger et al. 2004. Proc. Natl. Acad. Sci. USA 101:9891-9896). To date, the identity of all dw genes remains unknown.
The cultivation of tree species for agricultural production is expensive in large part because of manual architectural manipulations required to maximize productivity and harvesting efficiency. State-of-the-art pruning requires tremendous amounts of manual labor and costs. Harvesting fruit and nuts from large spreading trees often requires the use of ladders or elevated platforms. When using mechanized harvesting equipment such as shakers, substantial losses can be incurred from fruit bruising when dropped from high or dense canopies. Trees also require tremendous amounts of land space which leads to excessive use of fertilizers and pest control chemical inputs. As has been accomplished for cereals and other crops, genetic architectural improvements to enable high density production systems and mechanization stand to revolutionize the way in which fruits, nuts, and other tree-based crops are produced. Easier management will translate to immediate cost savings for growers and consumers. Developing crop trees with different degrees of dwarfing would lead to high density production, reduced manual labor costs, and reduced amounts of chemical (fertilizers and pest control) inputs needed and therefore a substantial cost savings and an environmental benefit. The ability to rationally manipulate tree size could also benefit the ornamental tree industry, enabling more aesthetically appealing landscape designs. To meet these challenges the development of improved varieties is vital.